1. Field of the Invention
The present invention relates to a power supply, and more particularly to a power supply having an extending hold-up time controlling unit.
2. Description of Related Art
Power systems are necessary elements for industrial equipments, and more specifically to an AC to DC power supply, which is one type of the power system. One important characteristic of the AC to DC power supply is hold-up time. When the line power is interrupted, the AC to DC power supply has to supply DC power for a while to let the load have enough time to storage data and orderly terminate the operation.
A general AC to DC power supply mainly has a rectifier, a power factor correction circuit, an output capacitor and a DC to DC converter. The output capacitor is coupled between outputs of the power factor correction circuit and inputs of the DC to DC converter. In the normal condition, the output capacitor has a function of suppressing ripple current. Once the AC power source supplied to the rectifier is interrupted, the output capacitor discharges a storage current therein to the DC to DC converter. The DC to DC converter outputs the DC power to the load for a while after the AC power source is interrupted, so the load shuts down safely.
Based on the foregoing description, a function of extending hold-up time is required by each power supply. Since a capacitance of the output capacitor determines the hold-up time, the output capacitor usually uses a bulk capacitor. However, the bulk capacitor has a large size to occupy a larger space in the power supply. Therefore, the bulk capacitor limits the miniaturization scale of the power supply.
To solve the foregoing drawback of the general AC to DC power supply, a hold-up time extension circuit is disclosed in the U.S. Pat. No. 6,504,497 (hereinafter '497 patent). With reference to FIG. 4, the hold-up time extension circuit (90) is electronically connected to a power supply having a rectifier (70), a diode (DBO), a DC to DC converter (80), an output capacitor (CBO) and an auxiliary capacitor (CBAUX). The rectifier (70) is coupled to the DC to DC converter (80) through the diode (DBO). The output capacitor (CBO) is electronically between the cathode electrode of the diode (DBO) and a ground. The auxiliary capacitor (CBAUX) is electronically between the anode electrode of the diode (DBO) and the ground. The end of the auxiliary capacitor (CBAUX) connected to the anode electrode of the diode (DBO) is also electronically connected to the hold-up time extension circuit (90). The output of the hold-up time extension circuit (90) is electronically connected to the input of the DC to DC converter (80).
When a voltage (VBO) stored in the output capacitor (CBO) is lower than a preset voltage, the auxiliary capacitor (CBAUX) discharges a storage current therein to the hold-up time extension circuit (90). The hold-up time extension circuit (90) converts the storage current of the auxiliary capacitor (CBAUX) and then outputs to the DC to DC converter (80). Therefore, the voltage of the output capacitor (CBO) is not decreased to zero in a short time and is extended.
Even thought the hold-up time extension circuit (90) of the '497 patent still has obvious drawbacks as follows.
1. In the normal condition, the output capacitor and the auxiliary capacitor are electronically connected in parallel. However, a diode is electronically connected between the output and auxiliary capacitors, so a positive voltage (about 0.7V) dropped at the diode exists between the output and auxiliary capacitors. The 0.7V voltage results that an output current with a large ripple current from the rectifier is output to the output capacitor. Therefore, the operation life of the capacitor is decreased. That is, under a normal condition of the AC power source, the auxiliary capacitor and output capacitor can not share averagely, so the output capacitor has a shorter operation life and easily becomes useless. Accordingly, an operation life of the AC to DC power supply is also decreased.
2. The AC to DC power supply including the hold-up time extension circuit of the '497 patent still uses a bulk capacitor to be the output capacitor to increase its operation life under receiving the larger ripple current condition. Therefore, the AC to DC power supply including the hold-up time extension circuit of the '497 patent does not reduce the entire size wildly.
With reference to FIG. 5, a circuit for maintaining hold-up time while reducing a size of the bulk capacitor size and improving efficiency in a power supply is disclosed in the U.S. Pat. No. 7,061,212 (hereinafter '212 patent). The power supply with a maintaining hold-up time circuit has a bridge rectifier, a first stage boost converter (110), a second stage boost converter (120), and a DC to DC converter (130). The first and second stage boost converters (110, 120) are electronically in serial between the bridge rectifier and the DC to DC converter (130). Therefore, the capacitors (118, 128) of the first and second stage boost converters (110, 120) commonly receive the ripple current from the bridge rectifier and are used to supply power to the DC to DC converter (130) when the AC power source is interrupted.
The power supply of the '212 patent uses two stage boost converters (110, 120) to solve the drawbacks of the power supply with the hold-up time circuit of the '497 patent. However, the inductor (112, 122) and diode (116, 126) of the two stage boost converters (110, 120) cause power consumption in partial to decrease the power efficiency of the AC to DC power supply.